The present invention is related to alkoxy-functional room temperature vulcanizable silicone compositions. More particularly, the present invention is related to scavenger-free, alkoxy-functional room temperature vulcanizable silicone compositions having improved shelf-stability.
Polyalkoxysilyl-terminated polydiorganosiloxanes are the primary constituent in alkoxy-functional room temperature vulcanizable (RTV) silicone compositions. The polyalkoxysilyl-terminated polydiorganosiloxanes are typically prepared by an end-capping reaction between a silanol-terminated silicone polymer with an alkoxysilane. A byproduct of this endcapping reaction is an organic alcohol, such as methanol. Fast endcapping of the silanol-terminated silicone polymer with the alkoxysilane usually requires a catalyst, several of which are known in the art. Unfortunately, such endcapping catalysts, as well as the alcohol by-product of the endcapping reaction, can promote silicone redistribution reactions, wherein new and different silicone polymers, e.g., monoalkoxy-terminated silicones, are formed which do not cure upon exposure to moisture and a cure catalyst. The ultimate result is an RTV composition which is not shelf-stable. In addition, the presence of the endcapping catalyst and the alcohol by-product can cause the polyalkoxysilyl-terminated silicone polymer to substantially decrease in viscosity upon storage. Such viscosity loss over time may render an RTV composition difficult or impossible to use. Thus, the endcapping catalyst and the alcohol by-product must be removed from the endcapped silicone polymer in order to prevent the silicone redistribution reactions and the viscosity loss which adversely affect the shelf stability and usefulness of the final RTV silicone composition.
U.S. Pat. No. 4,863,992 to Wengrovius et al. (Wengrovius) is directed to an improved endcapping catalyst for use in making the polyalkoxysilyl-terminated polydiorganosiloxane. The catalyst disclosed in the Wengrovius patent is an acidic amine salt which promotes fast endcapping but subsequently decomposes to form inert products, such as carboxylic acid amides and/or esters, which are ineffective to decrease the viscosity of the polyalkoxysilyl-terminated silicone.
It is desirable to improve the Wengrovius RTV system by increasing the decomposition rate of the endcapping catalyst used therein, thereby resulting in a polyalkoxysilyl-terminated silicone polymer having higher viscosity and greater stability in viscosity over prolonged storage periods and an RTV composition having greater shelf stability.
The present invention is based on the discovery that an organic formate, preferably an alkyl formate, will accelerate the decomposition of the ammonium formate endcapping catalyst.